How does photosynthetic rate drop as E drops from water stress?
~/pecanpro 8 Nov. 08
In the simulations done recently, I found what changes in PS capacity (Vcmax25) would cause various fractional drops in E (that is, E/E0). I did other simulations to find what changes in Ball-Berry slope (mBB) would cause these fractional drops.
A good question is, How does actual photosynthetic rate change as E changes? I took the simulation data and plotted the fractional change in A (A/A0) vs. the fractional change in E (E/E0).
The first set of simulations were done with a moderate value of the Ball-Berry intercept (bBB = 0.02 mol m-2 s-1). This value of bBB caused a high residual E/E0 when PS capacity dropped to zero.
Here are the plots for A/A0 vs. E/E0. The first plot shows the changes when they are driven by decreases in Vcmax25:
This second plot shows the same when stress changes mBB in order to cut transpiration:
Obviously, the stress response via stomatal control is "much kinder" to actual photosynthetic rate, A, than is the stress response via decreasing PS capacity.
Also, the relative changes look close to universal, that is, nearly the same for different environmental conditions
Recall the key:
28/0.3/1000 is the base case for 28°C (air T, long-time avg. air T to which the leaf acclimated, and T of surrounding veg.), 0.3 = 30% relative humidity, and a light level of 1000 micromol m-2 s-1, half of full sun = avg. value of randomly oriented leaves)
20/0.3/1000 is at lower T (20°C 0 but same relative humidity and same light level.
28/0.5/1000 is at the original T but higher relative humidity, and the same light level.
28/0.3/600 is at the original T and relative humidity, but at a lower light level.
This brings up the question of how the water-use efficiency changes in both kinds of stress responses. The relative WUE (WUE/WUE0) is easily calculated as (A/A0)/(E/E0). Here is the plot for changes achieved by changes in PS capacity:
Clearly, WUE drops, and rather drastically at high stress. This is because photosynthesis cuts back faster than does E.
The case of changes in E driven by changes in Ball-Berry slope is a big contrast:
Yes, WUE rises with stress, which is the classic (short-term) response. The behavior is still fairly close among different environmental conditions.
A big question is ecological and evolutionary: If WUE rises with stress when stomatal control is the means of controlling E, but it drops when changes in photosynthetic capacity are used to control E, why does the latter response ever occur? It appears to be maladaptive. My only guess at the moment is that it might lead to better use of nitrogen in the plant – not in the leaves that display this change, but in new leaves that come out after the stress ends, leaves that will get N that has been moved out of the stressed leaves. I hope to look for the balance between the two stress responses in long-term datasets at FluxNet sites.
The results are a bit "better" for both A and WUE when the leaf has a smaller residual conductance (bBB = 0.005 mol m-2 s-1):
Here is the plot of A/A0 vs. E/E0, when decreased E is driven by changes in PS capacity:
The intercept is a bit more favorable, but still disadvantageous. Also, the behavior is not as close to universal.
Here is the plot for changes in WUE:
The drop in WUE is less drastic for the first environmental condition, but almost unchanged for the other conditions.
Here are the changes when the Ball-Berry slope is changing under stress:
It looks similar to that for a larger residual stomatal conductance.